Usually, between a transmitter and a receiver, the carrier frequency used in transmission for the modulation of a signal and the frequency used on reception for the demodulation of this signal can never be strictly identical. A phenomenon of carrier-frequency offset is therefore necessarily observed between the transmitter and the receiver. Moreover, depending on the accuracy of the oscillators used in the transmitters and the receivers, this offset may be greater or lesser and degrades the reception of the signals if no correction is made.
The invention applies notably to low-consumption telecommunications in which the receivers are mobile devices of which the energy consumption must be limited, that is to say in which it is necessary to implement the simplest possible digital algorithms involving a minimum of operations. Moreover, for these same consumption reasons, but also for questions of cost, the oscillators used in these receivers may have low accuracy. A good estimation and an effective correction of the carrier-frequency offset are therefore indispensable.
The same applies to the frequency offset of the transmitted coded symbols which is however directly linked to the carrier-frequency offset. Specifically, in a transmission/reception device, the carrier frequency and the sampling frequencies are generated based on one and the same oscillator. Therefore, between a transmitter and a receiver, the relative offset of the carrier frequency and the relative offset of the frequency of the coded symbols are equal. The estimation of only one of these offsets, in the first place the carrier-frequency offset, is therefore sufficient.
A field in question is, for example, that of low energy-consumption communications using the direct sequence spread spectrum usually called DSSS. According to this principle, an information symbol is associated with a binary spread sequence of L bits called “chips”. These chips then modulate the carrier in phase or in frequency of a constant envelope signal. The communication is made in packet transmission mode and each packet comprises a preamble which is a predetermined sequence of DSSS coded symbols, this predetermined sequence being known to the receiver. This preamble therefore allows the synchronization of the receiver in order to demodulate a received packet.
The invention may therefore notably be applied in the particular field of wireless communications between a transmitter and a receiver the physical layer of which is as described in the standard IEEE 802.15.4 for the ISM (“Industrial, Scientific and Medical”) band at 2.4-2.48 GHz, using a modulation of the O-QPSK type.